The proposed research program is aimed at investigating three areas of cognitive development: children's existing knowledge, their ability to acquire new knowledge, and the more basic processes that may underlie developmental changes in the first two domains. With regard to existing knowledge, two issues are thought to be of particular interest: the states of knowledge that underlie different levels of task performance, and the order of emergence of various scientific reasoning skills. With regard to ability to acquire new knowledge, two other issues are thought to be crucial: the instructional conditions under which young children can acquire sophisticated sceintific concepts, and the issue of possible developmental differences in learning, the issue of readiness. Finally, regarding the more basic processes that may underlie development in the first two domains, the issues of what constitutes a rigorous developmental explanation and how we can provide evidence for such explanations will be given special attention. These issues will be explored with a wide age range of children, ranging from three-years through college age. Past research indicates that many of the scientific reasoning tasks under investigation have a desirable property for developmental research; even very young children know something about the tasks, while even adults do not know everything there is to know. Thus, the problems will allow us to observe development over an unusually protracted time period. A variety of theoretical perspectives will be brought to bear on the issues described above. The 16 proposed experiments are guided by ideas derived from Piagetian, information processing, the traditional learning models. All of these theories have contributed both to the content of the present approach and to the methodologies that will be used.